How We Calculate 3D Thermal Bridges

When a junction needs 3D

Many thermal bridges are linear — they run uniformly along their length, so a single 2D cross-section captures them. Others are genuinely three-dimensional and a flat section simply can't represent them:

• Three-way corners, where two walls meet a floor or roof — usually the coldest points in the whole envelope.
• Structural penetrations — a balcony slab, steel beam or column crossing the insulation line.
• Repeating point fixings — wall ties, and the brackets and rails that carry cladding and rainscreen systems.

For these, we build a full 3D model. (See our companion note on 2D thermal bridge calculations for the linear junctions.)

What the calculation gives you

For each 3D detail we report two numbers:

• Point thermal transmittance (χ-value, W/K) — the extra heat loss from a single localised bridge, over and above the surrounding wall, floor and any linear junctions already counted. Where a junction is three-dimensional but still extended, the 3D model is also used to derive an accurate Ψ-value (W/m·K).
• Temperature factor (fRsi) at the true coldest point. Because a three-way corner is colder than any of the 2D junctions that form it, the condensation and mould-growth check has to be made in 3D. It is assessed against the critical value in BRE IP 1/06 — 0.75 for dwellings, schools and residential buildings (higher for humid spaces).

These feed into the building's total thermal-bridge heat loss, which combines linear and point effects: H_TB = Σ(l · Ψ) + Σ(n · χ) — every metre of linear junction plus every occurrence of a point bridge.

Our method

We model each detail in COMSOL Multiphysics, following the procedure set out in BS EN ISO 10211:

1. Build the 3D assembly around the junction directly from your construction drawings, with every material layer and its measured thermal conductivity.
2. Apply the boundary conditions — internal and external temperatures, and the standard surface resistances from BS EN ISO 6946.
3. Set the model extent so the cut-off planes sit far enough from the junction that they don't influence the result.
4. Refine the mesh until the heat flow and surface temperature stop changing. A 3D mesh grows fast — each subdivision multiplies the number of cells — so we concentrate the detail around the bridge itself for a mesh-independent, reproducible result, as BR 497 requires.
5. Solve the steady-state heat-transfer field, then extract the 3D thermal coupling coefficient (L3D). Subtracting the plane-element and linear-junction contributions isolates the χ-value; the minimum internal surface temperature gives fRsi.
6. Apply UK conventions from BR 497 throughout — external dimensions, standard resistances and the agreed treatment of corner temperature factors — and, for metal cladding and rainscreen systems, the conventions in MCRMA TP18.

Our models are validated against the benchmark test cases in BS EN ISO 10211, so you can rely on the numbers.

The standards we follow

Core method

• BS EN ISO 10211 — numerical calculation of heat flows and surface temperatures at thermal bridges, including the 3D method and point thermal transmittances.
• BS EN ISO 6946 — thermal resistance of plane building elements and the standard internal/external surface resistances.

UK conventions

• BR 497 (BRE) — the UK conventions for calculating transmittance values and temperature factors consistently, including 3D corner temperature factors.
• BRE IP 1/06 — critical temperature factors for avoiding mould, and how linear and point junction losses carry into Building Regulations.

Condensation and ground heat loss

• BS EN ISO 13788 — assessing surface and interstitial condensation risk.
• BS EN ISO 13370 — heat transfer through the ground, for floor and below-ground junctions.

Glazing, framing and cladding

• BS EN ISO 10077-1 & -2 — thermal transmittance of windows and doors, including numerical modelling of frames.
• BS EN ISO 12631 — thermal transmittance of curtain walling.
• MCRMA TP18 — conventions for U-, f- and Ψ-values of metal cladding systems using 2D and 3D calculations, where point fixings dominate.

What you receive

A short technical report for each detail: the modelled geometry, the point thermal transmittance (and any linear Ψ-value), the temperature factor, a pass/fail against the relevant critical value, and a clear statement of the standards and conventions applied — everything a building control body or certifier needs to accept the result.